* Ex-VP named as coup leader, no reaction from him
* Gunfire in capital for 2nd day, estimated death toll rises
* U.S. urges citizens to leave country, U.N. urges restraint
By Andrew Green and Louis Charbonneau
JUBA/UNITED NATIONS, Dec 17 (Reuters) - The United Nations received reports from local sources in South Sudan on Tuesday that between 400 and 500 people had been killed and up to...